Add support for padding in the Snappy framed format.

This is specifically motivated by DICOM's demands that embedded data
must be of an even number of bytes, but could in principle be used for
any sort of padding/alignment needed.

R=sanjay


git-svn-id: http://snappy.googlecode.com/svn/trunk@82 03e5f5b5-db94-4691-08a0-1a8bf15f6143